The Eastern (Orthodox) and Western (Roman Catholic since 1054) Churches were all united and called Orthodox (“correct believing”) by the 4th century, but spilt from one another in 1054 in an event known as the Great Schism. Before the Schism, bishops of major cities, such as Rome, and Constantinople (now Istanbul), were equals. The Roman (Western) bishopric was founded by Peter, the “Rock” upon which Jesus would build his church; the Orthodox (Eastern) bishopric was founded by St. Andrew, the first-called Apostle. Each church believes that it is the original church, with the other breaking off. For the purposes of this article, the Eastern (Constantinopolitan) Church will be identified as “Orthodox” and the Western (Roman) Church will be identified as “Catholic.”
The Orthodox Church does not accept the Catholic idea, articulated at the First Vatican Council in the mid-19th century, of Mary’s Immaculate Conception—that she was born without original sin—though both churches believe that Mary remained a virgin after Jesus’ birth until her death. The Orthodox also reject the belief in purgatory.
Another point of contention between the churches is the matter of “filoque,” which means, in Latin, “and from the Son”—referring to the procession of the Holy Spirit. The Roman Church adopted this in its creed in 1009 but the Orthodox Church ultimately rejected it. Both churches practice baptism in the name of the Father, Son, and Holy Spirit from infancy on up, but confirmation, for Catholics, occurs only when a child (though adults are also confirmed) is able to express his or her belief; for the Orthodox, it occurs immediately after baptism.
Orthodox priests, unlike those in the Catholic Church, may be married, but bishops and above may not be. While the Catholic Church believes that its head, the Pope, is infallible under certain circumstances, the Orthodox do not credit their leader likewise.
The Orthodox make their sign of the cross a little differently from the Catholics. Though both, using their right hand, touch their forehead first, Catholics generally extend the index and middle finger while the Orthodox join their index and middle fingers to their thumb to signify the Trinity (the ring and pinky fingers are folded in toward the palm to symbolize Christ’s 2 natures). Both also touch the chest, but the Catholics then touch their left shoulder while the Orthodox touch their right shoulder first.
The most significant difference between Eastern and Western Christianity are doctrinal differences. A minor example of these otherwise complex differences is that Catholic priests are required to take a vow of celibacy and are not allowed to marry while Eastern Orthodox priests may be married. Anther difference is diversity. Western Christians are either Catholics or are from one of dozens of Protestant churches. Eastern Christians are almost exclusively Eastern Orthodox.
Catholics recognize the Church's hierarchy known as the "Magisterium" headed by the Pope, currently Pope Francis. Protestants largely reject the authority of the Pope and Magisterium. The Eastern Orthodox hierarchy is identical to the Catholics, except that the head is the Patriarch of Constantinople instead of the Pope.
Another major difference is liturgical language. Prior to Vatican II, all Catholic churches conducted Mass in Latin, and all Eastern Orthodox Churches said their masses in Greek. Vatican II changed the Catholic practice to allow masses to be performed in vernacular languages (i.e. the common language of the population where the church was situated), already a feature of most Protestant denominations. The Eastern Orthodox church retained its tradition of speaking Greek.